feat: add typed message tags

[bjarn]

Summary

• Add the Lettermint.MessageTag struct and new!/2 constructor.
• Add typed EmailBuilder.tags/2 support.
• Continue to accept atom-key and string-key maps.
• Keep EmailBuilder.tag/2 for the legacy single tag.

Validation

• Validate names and values without trimming or normalization.
• Reject reserved names and duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Update direct-send and pipe-operator README examples.

Verification

• mix format --check-formatted
• mix test
• Generated source check
• GitHub test matrix for supported Elixir and OTP versions